The effect of the onset of sympathetic innervation on the heart is difficult to study in the intact developing animal because other changes are occurring simultaneously. Alternatively, innervation can be studied in vitro by using a cell culture preparation. In this proposal primary cultures of trypsin-dispersed cells from newborn rat hearts and cocultures of these cells with neurons from the paravertebral sympathetic chain will be employed to examine the influence of sympathetic innervation on the interrelation of electrophysiological properties (determined by intracellular microelectrode recording) and chronotropic response to alpha and beta adrenergic agonists. Non-innervated (control) heart cultures will be thoroughly characterized by identifying mean values of action potential parameters, characteristics of cell coupling and spontaneous rhythm and concentration-response relations to alpha and beta agonists in the presence and absence of antagonists. Heart cultures will then be grown in the presence of specific chemical agents or hormones, the elctrophysiologic characteristics which are altered by this treatment will be identified and the effect of the treatment on pharmacologic responsiveness will be determined. The experiments with chemical and hormonal interventions will provide information on how pharmacologic response depends on electrophysiologic properties, add to our knowledge of possible effects of hormone imbalances in vivo and permit the perfecting of protocols prior to investigating the more complicated innervated cultures. Co-cultures of heart and nerve will then be compared to the non-innervated control cultures. This will permit the identification of the specific trophic effects of sympathetic neurons and the extent to which these effects alter pharmacologic responsiveness. The culture technique also permits the further investigation of the factors responsible for these trophic influences. Heart cultures can be grown in the presence of nerve-conditioned medium or norepinephrine to determine if any of the effects of innervation are chemically mediated and, if so, whether the chemical mediator is a neurotransmitter.